High yield method for preparing silicon nanocrystals with chemically accessible surfaces

ABSTRACT

Silicon nanocrystals with chemically accessible surfaces are produced in solution in high yield. Silicon tetrahalide such as silicon tetrachloride (SiCl 4 ) can be reduced in organic solvents, such as 1,2-dimethoxyethane (glyme), with soluble reducing agents, such as sodium naphthalenide, to give halide-terminated (e.g., chloride-terminated) silicon nanocrystals, which can then be easily functionalized with alkyl lithium, Grignard or other reagents to give easily processed silicon nanocrystals with an air and moisture stable surface. The synthesis can be used to prepare alkyl-terminated nanocrystals at ambient temperature and pressure in high yield. The two-step process allows a wide range of surface functionality.

RELATED APPLICATIONS

[0001] This application claims the benefit of the filing date of U.S.Provisional Patent Application Serial No. 60/323,285, filed on Sep. 19,2001, entitled “HIGH YIELD METHOD FOR PREPARING SILICON NANOCRYSTALSWITH CHEMICALLY ACCESSIBLE SURFACES” by Kauzlarich et al, the entiretyof which is incorporated herein by reference.

GOVERNMENT RIGHTS

[0002] The subject matter described herein was supported in part by NISTAdvanced Technology Program, Contract No. 70 NANBOH3028. The U.S.Government has certain rights in this invention.

TECHNICAL FIELD

[0003] The invention generally relates to silicon nanocrystals. Moreparticularly, the invention relates to silicon nanocrystals havingchemically accessible surfaces and methods of their preparation.

BACKGROUND OF THE INVENTION

[0004] There has been relatively little research into the synthesis ofsilicon nanocrystals via solution methods despite the interestingoptical and electronic properties that make them important for futuretechnological applications. Silicon nanocrystals exhibit visibleluminescence. The wavelength of the luminescence is proportional to thesize of the nanocrystal. Since silicon is an important electronicmaterial with applications ranging from computer chips to photovoltaics,nanoelectronics based on silicon has tremendous potential. In addition,since silicon is a biocompatible element, there are many possibleapplications in the field of biology and medicine as an inorganicfluoresecent probe, a biosensor, or a drug delivery agent. The largestbarrier to the utilization of nanocrystalline silicon is the lack of ahigh yield synthetic method that gives rise to good quality siliconnanocrystals.

[0005] Several possible methods for producing silicon nanoparticles havebeen attempted. These methods include the gas phase and solutiondecompostion of silanes, the reactions of silicon Zintl salts withsilicon halides as well as the solution reduction of silicon halides bysodium, lithium naphthalenide or hydride reagents or reduction ofSi(OEt)₄ with sodium. While some of these methods are solution reductionmethods, it has been suggested that both high temperatures and pressuresachieved by bomb reactions, ultrasonication, or annealing aftersynthesis are required to generate crystalline silicon. In addition,these techniques often give rise to surface oxide contamination.

SUMMARY OF THE INVENTION

[0006] It has now been discovered that crystalline silicon nanoparticleswith well-defined crystal facets and chemically accessible surfaces canbe produced at ambient temperatures and pressures in a simple one ortwo-step solution synthesis.

[0007] In one aspect, the invention generally features a method forproducing silicon nanocrystals. In one embodiment, the method includesthe steps of: contacting a silicon halide and a first reducing agent ina first organic solvent to produce halide-terminated siliconnanocrystals; and contacting the halide-terminated silicon nanocrystalsand a second reducing agent along with a preselected termination groupin a second organic solvent to produce silicon nanocrystals terminatedwith the preselected termination group. The first and second reducingagents may be identical. The first and second organic solvents may beidentical. The second reducing agent may also act as a terminatinggroup.

[0008] In another aspect, the invention generally features siliconnanocrystals. In one embodiment, the silicon nanocrystals have a sizedistribution wherein at least 95% of the silicon nanocrystals arebetween about 40 nm and about 80 nm and at least 80% of the siliconnanocrystals are between about 50 nm and about 70 nm.

[0009] In yet another aspect, the invention generally features a methodfor producing silicon nanocrystals in high yield. In one embodiment, themethod includes the step of reducing a silicon halide with a reducingagent in an organic solvent to produce halide-terminated siliconnanocrystals.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 schematically illustrates one embodiment of synthesis ofsilicon nanocrystals.

[0011]FIG. 2 shows transmission electron microscope and selected areaelectron diffraction images of silicon nanocrystals synthesizedaccording to an embodiment of the invention.

[0012]FIG. 3 shows scanning electron microscope and transmissionelectron microscope images of a silicon nanocrystal synthesizedaccording to an embodiment of the invention.

[0013]FIG. 4 shows an atomic force microscope image of a siliconnanocrystal synthesized according to an embodiment of the invention.

[0014]FIG. 5 shows a high resolution transmission electron microscopeimage of silicon nanocrystals synthesized according to an embodiment ofthe invention.

[0015]FIG. 6 shows a high resolution transmission electron microscopeimage of silicon nanocrystals synthesized according to an embodiment ofthe invention.

[0016]FIG. 7 shows a ²⁹Si{¹H} CP MAS NMR of silicon nanocrystalssynthesized according to an embodiment of the invention.

DESCRIPTION

[0017] In general, the disclosed method is a two-step synthesis,although a one-step process can be designed for synthesis ofnanocrystals terminated with certain functional groups.

[0018] As illustrated in FIG. 1, a silicon tetrahalide such as silicontetrachloride (SiCl₄) is first reduced in an organic solvent with asuitable reductant to produce halide-terminated silicon nanocrystals.The halide-terminated silicon nanocrystals thus obtained can then befunctionalized (i.e., terminated) with a desired ligand.

[0019] The first step is typically conducted at room temperature andunder atmospheric pressure. Any solvent can be used as long as itprovides the desired solubility and is inert to the reaction carried outtherein. Illustrative examples of such solvents include polyethers suchas 1,2-dimethoxyethane (glyme), 2-methoxyethylether (diglyme),triethyleneglycoldimethylether (triglyme) and other polyethers of theform MeO(CH₂CH₂O)_(n)Me. Other illustrative examples of solvents includetetrahydrofuran, 1,4-dioxane, aromatic solvents (e.g., benzene andtoluene), and alkanes (e.g., hexane).

[0020] Silicon halides that can be used as starting material includewhere the silicon halide is SiX₄, a mixed silicon halide,R_(n)SiX_((4-n)), or a mixture thereof, wherein X is a halide, R is analkyl group, and n=0, 1, 2, or 3. Mixtures of two or more of thesehalides can also be used. In addition, the disclosed method can also beapplied to elements other than silicon such as EX₄, where E is any group4 element and X is a halide. For example, SiBr₄ can be reduced with areductant in a solvent and then the resulting bromine terminatednanocrystal terminated as described below. A mixed silicon halide mayalso be used in the same synthetic scheme.

[0021] Many reductants can be used in the reduction of silicontetrahalide to silicon (SiCl₄ to Si having a potential of −0.24V).Illustrative reducing agents that can be used as the first and/or thesecond reducing agent include elemental metals such Li, Na, K, Rb, Cs,Mg, Ca, Zn, Al in either bulk or finely divided forms or as a liquidalloy such as Na/K or Hg/Na, Hg/K, Hg/Li, Zn, Al or alkali metals withphase transfer catalysts such as crown ethers; compounds of these metalssuch as naphthalenides, anthracenides, or acenaphthalenides or otherconjugated aromatic anions of Li, Na or K; compounds of these metalssuch as alkyl lithiums, alkyl aluminiums, alkyl zincs; metal hydridessuch as CaH₂, KH and NaH or LiAlH₄; Grignard reagents, various activatedforms of magnesium and other organomagnesium reagents such as magnesiumanthracenide. The first and second reducing agents can be different oridentical. Furthermore, more than one reducing agent can be used as thefirst or the second reducing agent.

[0022] In the second step, which is also typically conducted at roomtemperature and under atmospheric pressure, a large number of surfacetermination groups can be introduced to silicon nanocrystals. Forexample, the atom connecting to the silicon can be carbon to givehydrocarbon termination, hydrogen to give hydride termination, oxygen togive alkoxide termination, nitrogen to give amine termination, or sulfuror any number of other heteroatoms. Beyond the connecting atom, thenature of the termination group can vary. Any organic or inorganic groupcan be envisaged including termination groups containing functionality.These functionalities can then be further modified using conventionalorganic or inorganic chemistry to produce more complex chemical surfacesincluding oligomeric or polymeric groups.

[0023] In one embodiment, the second reducing agent also provides thepreselected termination group. For example, an alkyllithium (e.g.,n-butyllithium) may be used both as a reducing agent and as a source ofalkyl termination groups. Other reducing agents that may providetermination groups include alkoxides (e.g., butoxide) as a source ofalkoxide termination groups, anionic amine compounds (e.g., lithiumbis(trimethylsilyl)amine) for amino terminating groups, and metalhydrides (e.g., diisobutylaluminium hydride) for hydride terminatinggroups.

[0024] Illustrative examples of alkyl termination include termination bya substituted or unsubstituted alkyl group such as a methyl group, anethyl group, a propyl group, a butyl group, a pentyl group, and a hexylgroup. Illustrative examples of alkoxide termination include terminationby a substituted or unsubstituted alkoxy groups such as a methoxy group,an ethoxy group, a propoxy group, a butoxy group, a pentoxy group, and ahexoxy group. Illustrative examples of amine termination includetermination by a substituted or unsubstituted amine such as a dimethylamino group.

[0025] In one embodiment, tetrahedral shaped silicon nanocrystals areproduced by sodium naphthalenide reduction of silicon tetrachloride in1,2-dimethoxyethane followed by surface termination with an excess ofn-butyllithium. The preferred stoichiometry of the reaction iscalculated such that after complete consumption of the sodium, thesilicon:chlorine ratio would be 4:1. Without being bound to the theory,an active Si—Cl surface should be left on the particles formed, whichallows surface termination with n-butyllithium. The use of sodiumnaphthalenide rather than bulk sodium ensures homogeneity of thereaction mixture. Both the ¹H and ¹³C NMR spectra displayed broadresonances in a region consistent with the presence of n-butyl groups aswell as aromatic resonances due to surface naphthalene moieties.

[0026] Hexane solutions of these particles showed no evidence ofphotoluminesence attributable to silicon nanocrystal quantumconfinement. For such effects to be observed, the particles need to besubstantially smaller.

[0027] Without being bound to the theory, a Wurtz type couplingmechanism can be used to explain the oligimerization of silicon chlorideprecursors into a nanocrystal. Sodium naphthalenide acts as a solubleand active source of sodium metal. It can exchange alkali metal forhalide on silicon. The resulting species can then react with anothersilicon halide to give a silicon silicon bond. This can explain both theintermolecular formation of Si—Si bonds needed to add an atom to thegrowing cluster, as well as the intramolecular bond formation requiredto close the six membered rings of silicon diamond structure.

[0028] In one embodiment, more than 80% of the silicon nanocrystalsproduced are tetrahedral in shape. In another embodiment, more than 90%of the silicon nanocrystals produced are tetrahedral in shape. In yetanother embodiment, more than 95% of the silicon nanocrystals producedare tetrahedral in shape. Again, without being bound to the theory, intetrahedral crystals of a diamond cubic crystal type, the (111) planemust give rise to the facets. For a given single crystal, the mostprevalent faces will be those that grow slowest. The face with thegreatest surface roughness and largest number of dangling bonds willgive rise to the fastest growth, while the smoothest with the leastnumber of dangling bonds will have the slowest face growth and thus bethe most prevalent face. In the case of diamond crystalline silicon,choice of facet greatly changes the number of dangling bonds. Cleavagethrough the (111) plane gives rise to surface silicon atoms with onlyone dangling bond whereas through other planes surface silicon atomswith more dangling bonds are observed. The slow growth of the (111)plane is consistent with this.

[0029] In one embodiment, silicon nanocrystals are produced by thesodium naphthalide reduction of SiCl₄, followed by termination with1-octanol. The particles thus produced have a mean diameter of 5.2±1.2nm. They are crystalline and faceted in the high resolution TEM. Thesolution ¹H NMR and IR are both consistent with octanoxide groups on thesurface. These particles are small enough to exhibit photoluminescence.For an excitation at 320 nm emission is observed in the 410-420 nmrange. ²⁹Si{¹H} CP MAS NMR of these nanocrystals, as well as energydispersive x-ray spectroscopy confirm the nanocrystals are silicon.

[0030] Any solvent may be used in the second step of the synthesis aslong as the desired silicon nanocrystals are produced. Illustrativeexamples of such solvents include polyethers such as 1,2-dimethoxyethane(glyme), 2-methoxyethylether (diglyme), triethyleneglycoldimethylether(triglyme) and other polyethers of the form MeO(CH₂CH₂O)_(n)Me. Otherillustrative examples of solvents include tetrahydrofuran, 1,4-dioxane,aromatic solvents (e.g., benzene and toluene), and alkanes (e.g.,hexane). In certain embodiments, identical solvent(s) is employed in thefirst and the second steps. A single or a mixture of solvents may beused in the first and/or the second step of the synthesis.

[0031] In one embodiment, the silicon nanocrystals that are produced bythe disclosed method typically range from about 1 nm to about 100 nm.The size of the nanocrystals and the size distribution can becontrolled. In one detailed embodiment, 95% of the nanocrystals fallwithin the 1 nm to 80 nm range and 80% fall within 1 nm to 70 nm range.In another detailed embodiment, 80% of the nanocrystals fall within 1 nmto 50 nm range. In yet another detailed embodiment, 80% fall within 1 nmto 10 nm range.

[0032] The yield of the silicon nanocrystals is more than 90% andtypically is more than 95% by weight of silicon in the startingmaterial. Quantitative transformation from silicon tetrahalide tosilicon nanocrystals can also be achieved.

[0033] Silicon nanocrystals, such as butyl-terminated siliconnanocrystals, prepared according to the disclosed method are free fromsurface contamination and are air and moisture stable. In oneembodiment, silicon nanocrystals prepared are stable at ambient air andmoisture conditions for at least one day. In another embodiment, siliconnanocrystals prepared are stable at ambient air and moisture conditionsfor at least one week.

[0034] In one embodiment, the synthesis can be completed in a singlestep. For example, a mixture of R_(n)SiX_((4-n)) (e.g., t-BuSiCl₃) andSiX₄ (e.g., SiCl₄), where R is an alkyl group, X is a halide, and n is0, 1, 2, or 3, can be reduced in a solvent with a reductant. The alreadysubstituted t-BuSiCl₃ provides the termination groups on reduction whilethe SiCl₄ provides the crystalline core of the silicon nanocrystal onreduction.

[0035] The advantages of the disclosed method over the current state ofthe art include the chemical accessibility of the chloride termination,allowing easy access to different terminating groups, assilicon-chloride bonds can easily be replaced by silicon-other elementbonds. The advantages also include reaction conditions of ambienttemperature and pressure, the easy increase in scale, and the ability tocontrol the yield of the silicon nanocrystals formed. The flexiblenature of the synthetic procedure also allows control of the size andshape of the nanocrystals formed. For instance, reduction of SiCl₄ withsodium naphthalenide followed by termination with BuLi generates largetetrahedral silicon nanocrystals while reduction of silicontetrachloride with sodium naphthalenide followed by termination withoctanol gives rise to substantially smaller silicon nanocrystals of lesswell defined shape. The silicon halide starting material, especiallysilicon tetrachloride, is relatively inexpensive and readily available.In addition, such an easily manipulated solution route allowspreparation of doped silicon nanocrystals as a great variety ofpotential dopants can be readily introduced in a solution.

EXAMPLES

[0036] The following examples are provided to further illustrate and tofacilitate the understanding of the invention. These specific examplesare intended to be illustrative of the invention. Two examples of thesolution reduction methods at ambient temperature and pressure for theproduction of silicon nanocrystals are described here.

Example 1

[0037] A tetrahydrofuran solution of sodium naphthalenide (0.9 g, 39mmol Na and 3.23 g, 25 mmol of naphthalene in 30 cm³ tetrahydrofuranstirred for three hours) was added rapidly at room temperature viacannula to a 500 cm³ Schlenk flask containing 300 cm³ of1,2-dimethoxyethane and 1.14 g, 6.7 mmol of SiCl₄ with rapid stirring.On completion of the naphthalenide addition a large excess ofn-butyllithium (10 cm³, 1.6M in hexane) was added immediately. Thesolvent was removed under vacuum from the resulting yellow brownsuspension. The residue was extracted with hexane and washed with waterto remove sodium and lithium salts. Evaporation of the hexane layerfollowed by heating under vacuum to remove residual naphthalene gave 0.7g of a viscous yellow oil.

[0038] Transmission electron microscopy (TEM) shows the presence ofcrystalline silicon tetrahedra of approximately 60 nm in edge length.The bright-field TEM image of a number of the silicon nanocrystals on aholey carbon grid, along with the selected area electron diffraction(SAED) pattern is given in FIG. 2. In FIG. 2, darkness is relative toelectron beam opacity of the sample. To prepare the TEM grids the samplewas diluted approximately 100 fold in hexane and sonicated to resuspendthe nanocrystals. A 20 μL aliquot of this solution is dropped on a holeycarbon grid which was then dried in an oven at 120° C. for 1 h. The darktriangular shapes are the silicon nanocrystals. The particles arefaceted with tetrahedral morphology. The SAED (inset in the top left ofFIG. 2) spot pattern is consistent with the silicon diamond lattice. Asurvey of fifty-five particles from several different areas on the gridgave edge lengths between 40 and 130 nm with 95% of the particlesfalling within the 40-80 nm range and 80% falling within the 50-70 nmrange.

[0039] Solution NMR spectroscopy of the resulting yellow oil wasconsistent with surface alkyl termination. This is also confirmed byscanning electron microscopy (SEM), FIG. 3, which shows larger SEM imagethan those in TEM, consistent with the nanocrystals being covered withan organic layer that images in the SEM but does not image in the TEM.In FIG. 3, the left image is a SEM image of a silicon nanocrystal whilethe right image is the TEM image of the same nanocrystal at the samemagnification. It can be seen that the TEM image is much smaller thanthe SEM image. The three-dimensional geometry of the siliconnanocrystals are confirmed by atomic force microscopy (AFM). FIG. 4shows an AFM topograph of a typical nanocrystal.

Example 2

[0040] Sodium naphthalenide, prepared from sodium (0.69 g) andnanphthalene (2.39 g) stirred in 70 cm³ of 1,2-dimethoxyethaneovernight, was added rapidly via cannula to a stirred solution of 1.04 gof SiCl₄ in 1,2-dimethoxyethane. The dark brown suspension obtained wasstirred for a further 30 minutes then 5 cm³ of 1-octanol was added. Ayellow solution with a white precipitate was observed immediately. Thesolvent and naphthalene were removed immediately with heating in a waterbath. The resulting orange solid was extracted with hexane and washedthree times with slightly acidic distilled water. The hexane layer wascollected and pumped down to give a waxy orange hexane soluble solid.Solution ¹H NMR was consistent with octanoxide groups on the surface ofthe nanocrystals. This was also supported by the IR spectrum in which aSi—OR stretch was observed at ˜1080 cm⁻¹ and an alkyl C—H stretch at3000 cm⁻¹. The nanocrystals

[0041] Holey carbon grids for TEM were prepared by suspending thenanocrystals in hexane with ultrasonication, then dipping the grid intothe suspension thus obtained and then allowing the solvent to evaporate.FIG. 5 gives a high resolution TEM image of an area on the grid alongwith a histogram of sizes. The dark spots are nanocrystals. The meandiameter of the observed particles was found to be 5.2±1.9 nm from asurvey of 718 nanocrystals. FIG. 6 gives a high resolution image of alarge silicon nanocrystal. The particle is crystalline and faceted. Thelattice fringes of the particle are clearly visible, the spacing of0.314 nm consistent with the <111> plane of diamond crystalline silicon.Energy dispersive x-ray spectroscopy also confirms the presence ofsilicon, as does solid state ²⁹Si{¹H} CP MAS NMR (FIG. 7).

Example 3

[0042] Magnesium powder (0.47 g) was suspended with sonication in 70 cm³of dry 1,2-dimethoxyethane. To this was added 1.18 cm³ of SiCl₄ and theresulting suspension sonicated overnight. To the red/brown solution andgray precipitate thus obtained was added 2 cm³ of 20% MgBuCl intetrahydrofuran. The solvent and any other volatiles were removed undervacuum, the resulting colorless oil taken up in hexane, washed withslightly acidified water and again pumped down to a colorless oil. Theoil contained small silicon nanoparticles by TEM (<20 nm). ¹H NMR and IRconfirmed the existence of terminating butyl groups.

Example 4

[0043] Naphthalene (3.061 g) and sodium (0.8 g) were stirred in 60 cm³of dry 1,2-dimethoxyethane for 2 days. The mixture was then added to asolution of 0.58 g of SiCl₄ and 0.65 g of t-BuSiCl₃ in 300 cm³ of1,2-dimethoxyethane at 0° C. over a period of three minutes. A deepyellow/brown solution was obtained. Glyme and naphthalene were removedunder reduced pressure and the resulting oil extracted with hexane. ¹HNMR was consistent with the presence of terminating t-butyl groups whilethe TEM showed crystalline silicon nanoparticles of a range of sizes.

[0044] Variations, modifications, and other implementations of what isdescribed herein will occur to those of ordinary skill in the artwithout departing from the spirit and the scope of the invention.Accordingly, the invention is not to be limited only to the precedingillustrative descriptions.

What is claimed is:
 1. A method for producing silicon nanocrystalscomprising the steps of: contacting a silicon halide and a firstreducing agent in a first organic solvent to produce halide-terminatedsilicon nanocrystals; and contacting the halide-terminated siliconnanocrystals and a second reducing agent along with a preselectedtermination group in a second organic solvent to produce siliconnanocrystals terminated with the preselected termination group.
 2. Themethod of claim 1 wherein the silicon halide is SiX₄, a mixed siliconhalide, R_(n)SiX_((4-n)), or a mixture thereof, wherein X is a halide, Ris an alkyl group, and n=0, 1, 2, or
 3. 3. The method of claim 1 whereinat least one of the first and the second reducing agents is an elementalmetal having an oxidation potential greater than 0.24V.
 4. The method ofclaim 3 wherein the elemental metal is in bulk form.
 5. The method ofclaim 3 wherein the elemental metal is in finely divided form.
 6. Themethod of claim 3 wherein the elemental metal is selected from the groupconsisting of Li, Na, K, Rb, Cs Mg, Ca, Zn, and Al.
 7. The method ofclaim 1 wherein at least one of the first and the second reducing agentscomprises a liquid alloy comprising one metal component that has anoxidation potential greater than 0.24V.
 8. The method of claim 7 whereinat least one of the first and the second reducing agents comprises anagent selected from the group consisting of a liquid alloy of Na/K,Hg/Na, Hg/K, Hg/Li, Hg/Zn, and Hg/Al.
 9. The method of claim 1 whereinat least one of the first and the second reducing agents comprises amixture of an alkali metal and a phase transfer catalyst.
 10. The methodof claim 1 wherein the phase-transfer catalyst is a crown ether.
 11. Themethod of claim 1 wherein at least one of the first and the secondreducing agents comprises an aromatic anion, an alkyl metal compound, ora metal hydride of an metal selected from the group consisting of Li,Na, K, Mg, Ca, Zn, and Al.
 12. The method of claim 1 wherein at leastone of the first and the second reducing agents comprises an activatedmagnesium reagent.
 13. The method of claim 1 wherein the first and thesecond reducing agents are the same.
 14. The method of claim 1 whereinat least one of the first and second organic solvents comprises apolyether having the formula of MeO(CH₂CH₂O)_(n)Me.
 15. The method ofclaim 14 wherein the polyether is selected from the group consisting of1,2-dimethoxyethane (glyme), 2-methoxyethylether (diglyme), andtriethyleneglycoldimethylether (triglyme).
 16. The method of claim 1wherein at least one of the first and second organic solvents comprisestetrahydrofiran, 1,4-dioxane, benzene, toluene, or hexane.
 17. Themethod of claim 1 wherein the first and second organic solvents are thesame.
 18. The method of claim 1 wherein the preselected terminationgroup comprises an alkyl termination group, a hydride termination group,an alkoxy termination group, an amino termination group, or a mixturethereof.
 19. The method of claim 18 wherein the preselected terminationgroup is an oligomeric or polymeric group.
 20. The method of claim 1wherein at least 95% of the silicon nanocrystals are between about 1 nmto about 80 nm and at least 80% of the silicon nanocrystals are betweenabout 1 nm to about 70 nm.
 21. The method of claim 1 wherein at least95% of the silicon nanocrystals are between about 40 nm to about 80 nmand at least 80% of the silicon nanocrystals are between about 50 nm toabout 70 nm.
 22. The method of claim 1 wherein at least 80% of thesilicon nanocrystals are between about 1 nm to about 50 nm.
 23. Themethod of claim 1 wherein at least 80% of the silicon nanocrystals arebetween about 1 nm to about 10 nm.
 24. The method of claim 1 wherein atleast one of the steps of contacting a silicon halide and a firstreducing agent in a first organic solvent and contacting thehalide-terminated silicon nanocrystals and a second reducing agent alongwith a preselected termination group is carried out at ambienttemperature and pressure.
 25. The method of claim 1 wherein both stepsof contacting a silicon halide and a first reducing agent in a firstorganic solvent and contacting the halide-terminated siliconnanocrystals and a second reducing agent along with a preselectedtermination group are carried out at ambient temperature and pressure.26. The method of claim 1 wherein the steps of contacting a siliconhalide and a first reducing agent in a first organic solvent andcontacting the halide-terminated silicon nanocrystals and a secondreducing agent along with a preselected termination group are carriedout concurrently in one reaction.
 27. The method of claim 1 wherein theyield of silicon nanocrystals is greater than 90% by weight of siliconin the staring silicon halide.
 28. The method of claim 1 wherein thesecond reducing agent provides the preselected termination group. 29.The method of claim 28 wherein the second reducing agent is alkyllithiumand the preselected termination group is an alkyl group.
 30. The methodof claim 29 wherein the alkyllithium is n-butyllithium.
 31. The methodof claim 28 wherein the second reducing agent comprises a metalalkoxide, an anionic amine, or a metal hydride.
 32. Silicon nanocrystalshaving a size distribution wherein at least 95% of the siliconnanocrystals are between about 40 nm to about 80 nm and at least 80% ofthe silicon nanocrystals are between about 50 nm to about 70 nm.
 33. Thesilicon nanocrystals of claim 32 wherein at least 80% of the siliconnanocrystals are between about 50 nm to about 60 nm.
 34. The siliconnanocrystals of claim 32 wherein substantially all the siliconnanocrystals are tetrahedral shaped.
 35. The silicon nanocrystals ofclaim 32 having surface termination in the form of an alkyl terminationgroup, an alkoxy termination group, an amino termination group, ahydride terminating group or a mixture thereof.
 36. A method forproducing silicon nanocrystals in high yield comprising the step ofreducing a silicon halide with a reducing agent in an organic solvent toproduce halide-terminated silicon nanocrystals.
 37. The method of claim36 further comprising the step of reacting the halide-terminated siliconnanocrystals with a preselected termination group to form siliconnanocrystals terminated with the preselected termination group.
 38. Themethod of claim 37 wherein the step of reacting the halide-terminatedsilicon nanocrystals with a preselected termination group comprisescontacting the halide-terminated silicon nanocrystals and a reducingagent along with a reagent containing the preselected termination groupin an organic solvent.
 39. The method of claim 37 wherein thepreselected termination group is an alkyl group, an alkoxy group, anamino group, a hydride group, or a mixture thereof.
 40. The method ofclaim 36 wherein the silicon halide comprises R_(n)SiX_((4-n)) and SiX₄,wherein R is an alkyl group, X is a halide, and n is 0, 1, 2, or
 3. 41.The method of claim 40 wherein R is t-butyl, X is Cl, and n=1.